Laserfiche WebLink
AVCOLORADO <br />Division of Water Resources <br />Department of Natural Resources <br />MEMORANDUM <br />To: <br />Bill Tyner <br />From: <br />Matt Sares <br />Date: <br />6/26/2019 <br />Subject: Mayhoffer East Farm - Interruptible Water Supply Lagging Review <br />The Mayhoffer East Farm (Farm) is the proposed location of an Interruptible Water Supply <br />Agreement. It is located near La Junta, Colorado, about one mile south of the Arkansas River <br />in T24S, R55W, Sec 8 NE -NE. The question posed is: How would lagging from farm irrigation <br />accrue to the Arkansas River and its alluvium, via groundwater or surface water pathways? <br />The only geologic map available for this location is the La Junta 1x2°map (Scott, 1968; scale <br />1:250,000). This map indicates the Farm is located on a small "island" deposit of Slocum <br />alluvium (Qs) overlying outcrop of the Fort Hays Limestone Member of the Niobrara Formation <br />(Figure 1). On the Farm, Slocum alluvium would readily absorb water from irrigation of crops, <br />but what happens as that water infiltrates to the base of the alluvium and encounters the Fort <br />Hays Limestone Member. It is important to note that the overlying Smoky Hill Member of the <br />Niobrara Fm, which is dominated by low permeability shale and platy limestone, is not mapped <br />in this location -- it has been removed by erosion and is not present. <br />In my opinion, irrigation water applied to the Farm would infiltrate into the Slocum alluvium <br />and then into the extensively fractured Fort Hays Limestone (Figure 2) . Within the Fort Hays, <br />thin shale beds separate some limestone beds, but these are likely also fractured. Low <br />permeability Carlile Shale underlies the Fort Hays Limestone and serves as a vertical <br />permeability barrier. Topography in this area slopes toward the Arkansas River alluvium, <br />therefore groundwater would move through the Fort Hays Limestone to the north, eventually <br />discharging to Arkansas River alluvium (both Broadway (Qb) and modern alluvium (Qat)). <br />Therefore, lagging from the Farm to the Arkansas River alluvium should be estimated for the <br />horizontal permeability found in the fractured Fort Hays Limestone. <br />In the possibility that the Fort Hays Limestone may be relatively impermeable in some areas, a <br />review of the soil units (above bedrock) along the pathway from the Farm to the Arkansas <br />alluvium was assessed (NRCS, Figure 3). Soil thicknesses indicate a range from 20 to more than <br />80 inches. All soil units along the likely path of percolated irrigation water are rated as "well <br />drained." (for details see custom soil report attached to email) <br />I will caveat the above interpretations with the fact that I am relying solely on published <br />geological literature, without the benefit of a field visit of the area. <br />1313 Sherman Street, Room 821, Denver, CO 80203 P 303.866.3581 www.colorado.2ov/water <br />Jared Polis, Governor Dan Gibbs, Executive Director Kevin G. Rein, State Engineer/ Director <br />